Plastics are classified as thermoplastic resins or as thermosetting resins, depending on how the plastic is affected by heat. When heated, thermoplastic resins soften and flow as liquids; when cooled, they solidify. These changes on heating and cooling can be repeated. Thermoplastics are thermally stable in their intended applications. Generally, thermoplastic products are made by melting thermoplastic compounds, and shaping and cooling the melt.
When heated, thermosetting resins liquify and then solidify with continued heating due to crosslinking of the plastic during heating. Crosslinking is a permanent change; once molded, a thermoset plastic cannot be reheated and remolded. In contrast, thermoplastic parts can be reworked.
Honeycombs are used as structural elements where a combination of lightness and strength is desirable. Plastics have been used in the manufacture of a variety of honeycomb structural elements such as panels. Polyimide film honeycomb panels are described in U.S. Pat. No. 4,921,745. Fiber reinforced polyimide honeycomb structures for high temperature applications are described in allowed U.S. patent application Ser. No. 08/261,377. Formable honeycomb panels are described in U.S. Pat. No. 4,859,517. Methods for making thermally fused thermoplastic honeycomb structures are described in U.S. Pat. No. 5,421,935. Multi-layer honeycomb structures are described in U.S. Pat. No. 5,106,688.
One problem with plastics such as thermoplastic urethane honeycomb panels is that it is difficult to adhere materials to the plastic. The present invention overcomes this and other problems.